Page 194 - Anatomy and Physiology of Farm Animals, 8th Edition

Microscopic Anatomy and Physiology of Muscle / 179

The concentration of CP is also Strength of Contraction
limited. Thus, if muscle contraction con-
VetBooks.ir tinues for longer than a few seconds, the Whenever a single muscle fiber receives a

CP and new ATP eventually have to be
reconstituted by the citric acid cycle nervous impulse and action potentials are
generated on the muscle fiber, the action
(Krebs cycle) in the mitochondria of the potentials will be propagated over the entire
muscle fibers. If muscle activity outstrips fiber and cause the whole fiber to contract.
the ability of the mitochondria to pro- This is the all‐or‐none law of muscle
duce ATP aerobically, anaerobic metabo- contraction.
lism of carbohydrate fuel begins and The all‐or‐none law applies to a single
lactic acid accumulates in the muscle muscle fiber or a single motor unit
cell. Glucose (the major carbohydrate (a motor neuron and all the muscle fibers it
fuel) is obtained from the blood supply supplies); it does not apply to an entire
to the muscle and from the glycogen muscle, such as the m. biceps brachii. The
stored in the muscle fibers. Glycogen is all‐or‐none law also does not state that a
broken down by glycogenolysis. Glycog- muscle fiber will always contract with the
enolysis and glycolysis are complex pro- same speed or the same force; rather, it
cesses involving a number of reactions, states that for the conditions at the time of
enzymes, and intermediate compounds. stimulation, the muscle fiber will contract
Oxygen from the blood must also be to its maximum. The force of contraction
supplied to the mitochondria in muscles does depend on the state of the fiber at the
for the citric acid cycle to operate and time, that is, whether it is fatigued,
result in oxidative phosphorylation of ADP stretched to its optimal length, and so on.
to ATP. While lactic acid was mostly being The sliding filament mechanism is possible
produced anaerobically during muscle because of overlap of the thin and thick
contraction, an oxygen debt was building filaments. This overlap permits the binding
up. This oxygen debt must be repaid during between actin and myosin. Experimental
relaxation before optimal muscle activity studies have demonstrated that the amount
can resume. of overlap of the filaments before contrac-
The chain of reactions involved in sup- tion begins affects the contraction strength
plying energy for muscle contraction and of individual muscle fibers. When muscle
recovery is shown in Table 9‐1. fibers are stretched before being stimu-
lated, contraction strength increases up to
an optimal amount of stretch. Any further
stretching produces a decrease in con-
Table 9-1. Chain of Reactions That Supply traction strength. This same relationship is
Energy for Muscle Contraction and Recovery true for the other striated muscle, cardiac
muscle, and is an important factor in the
ATP → ADP + phosphoric acid + energy
(for immediate use in regulation of cardiac contraction strength.
contraction)
CP → Creatine + phosphoric Summation. Each gross muscle, which is
acid + energy (for resynthesis of composed of multiple motor units and
ATP from ADP) many individual muscle fibers, is capable
Glucose (glycogen Lactic acid + energy (for of contracting with varying degrees of
or blood) → resynthesis of CP from creatine strength. This is the result of summing the
and phosphoric acid) contractions in two ways. Motor unit
Lactic Water + carbon summation (recruitment) occurs when
acid + oxygen → dioxide + energy (for more motor units are stimulated to contract
resynthesis of ATP and CP) simultaneously in the gross muscle. There-
ATP = adenosine triphosphate; ADP = adenosine fore, more muscle fibers and bundles are
diphosphate; CP = creatine phosphate. contracting and producing greater strength